Rotor Overspeed

• Background
• Causes
• Symptoms
• Emergency Procedure

Background

Most helicopters are equipped with turboshaft engines governed by electronic controllers. The controller aims to keep the rotors turning at a constant speed, counteracting the aerodynamic drag that would otherwise slow the rotors.

Causes

An engine overspeed may be caused by

• a failure measuring rotor speed,
• a failure in fuel flow monitoring or control,
• a communication failure of fuel flow, commands, rotational speeds, …,
• faulty controller/software, or
• a sudden change in torque load on the rotors.

Symptoms

Potential symptoms of a rotor overspeed follow.

• Cockpit indicators warning of high rotor speed (NR), power turbine speed (NP) or gas generator speed (NG). (Exceptions include quick stops, autorotations, or other flight conditions where the airloads themselves may power the rotor to high NR.) One or more of these speeds may be displayed incorrectly if the overspeed was caused by the associated speed detection equipment.
• A “torque split,” meaning the affected engine provides more torque than the unaffected engine. The unaffected engine automatically reduces fuel flow in attempt to maintain the original NR.
• An overspeed detection system may automatically cut off fuel to the affected engine.

Emergency Procedure

Throughout this emergency, the collective may be used as a backup mechanism for holding rotor speed. For example, if rotor speed drops as one engine is shutdown, then the collective should be lowered to achieve operational rotor speed.

If an engine is overspeeding and not being handled by an overspeed protection system, the engine should be shutdown with the appropriate throttle or switch depending on the helicopter model. If the engine does not respond to the appropriate throttle or switch, other switches or buttons that perform emergency shutdown could be used. For example, many helicopters have a FIRE PBA that, when depressed, cuts fuel to the engine.

Depending on the helicopter and flight condition, the next step will be to either land (OEI or autorotating) or attempt to restart the engine. When flying in the degraded condition, the flight envelope will be limited as indicated in the flight manual. Typically this will limit airspeed among other things.